[HN Gopher] Foucault pendulum
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Foucault pendulum
Author : bottle2
Score : 16 points
Date : 2020-04-27 12:45 UTC (10 hours ago)
(HTM) web link (en.wikipedia.org)
(TXT) w3m dump (en.wikipedia.org)
| empath75 wrote:
| > The pendulum was introduced in 1851 and was the first
| experiment to give simple, direct evidence of the earth's
| rotation.
|
| The existence of the day-night cycle should have sufficed.
| lidHanteyk wrote:
| In addition to sibling comments, we should note that not all
| observations are experiments. There is a distinct difference
| between experiment and mere observation: In an experiment, we
| are carrying out a repeatable recipe in order to try to falsify
| a hypothesis. We are not merely theory-crafting, working to
| explain what we see, but we are trying to disprove what we have
| theorized.
| jschwartzi wrote:
| And to clarify, observation is as important as experiment,
| because observation gives us new ideas for experiments.
| Collecting anecdotes and making observations is as important
| as designing experiments. You can't really be a scientist if
| you don't pay attention to the natural world.
| KMag wrote:
| The pendulum shows you the net rotation of the pendulum about
| the Earth's axis of rotation, the rotation of the Earth about
| the Sun, the Sun about the center of the Milky Way, the Milky
| Way about the gravitational center of our local cluster, etc.,
| etc.
|
| Having not known, I think I would have strongly guessed that
| the rotation of the Earth about its own axis would be by far
| the dominant factor, but it's still good to measure.
| [deleted]
| saagarjha wrote:
| That doesn't tell you whether the earth rotates or the sun
| revolves around the earth.
| jschwartzi wrote:
| Yes. People have been taught that the earth revolves around
| the sun, and sometimes we use it as a way to be smug about
| how much smarter we are than the ancients. But schools do a
| huge disservice to students by not showing them that there's
| not in fact any way to know this from the information the
| ancients had available to them. About all you can infer from
| the way the sun changes position in the sky is that some kind
| of movement is happening. Because the earth feels solid under
| your feet you might assume that only the sun, planets, and
| stars are moving. This is valid according to Occam's Razor.
|
| Indeed for hundreds of years western astronomers predicted
| the locations of the planets using epicycles and not by
| calculating orbits assuming a heliocentric system. And it
| worked! Beautifully! The earth-centric model has excellent
| predictive power. It explained everything the ancients were
| aware of.
|
| This is all as a lesson in both humility and in recognizing
| that our ancestors were not stupid or lacking reason. They
| just didn't have the tools we have. And for all that the
| heliocentric seems obvious, remember that it is only obvious
| because we have been taught it. Our ancestors had been taught
| an earth-centric model instead, and it was obvious to them.
| libraryofbabel wrote:
| To turn that on its head, it's a wonderful teaching moment
| to ask a room full of smart students to try and prove (or
| even convincingly argue) that the Earth moves around the
| sun. A great way to get people to examine their assumptions
| and look afresh at the world around them.
|
| As you say, it is indeed very hard to do, which is a good
| part of the reason why even when Copernicus published his
| heliocentric model (not a proof, but a new mathematical
| model) his ideas took decades to become mainstream. If you
| have a rudimentary telescope / binoculars and know to look,
| then you can observe that Venus has phases and use that as
| a basis for argument (as Galileo did), but even then, it's
| perfectly possible to cook up special models where Venus
| has phases and the earth is still stationary (see
| https://en.m.wikipedia.org/wiki/Tychonic_system).
| empath75 wrote:
| They certainly could and in fact did.
|
| https://en.wikipedia.org/wiki/Heliocentrism#Aristarchus_of_
| S...
| lurquer wrote:
| I found it very difficult to understand WHY Foucaults pendulum
| behaves the way it does.
|
| I could picture what was happening with the pendulum at the Norh
| Pole... earth rotates under the pivot point every 24 hours. And,
| I understand what happened st the Equator... no effect on the
| pendulum as nothing is rotating beneath it.
|
| But, at points in between, it was hard to understand why the
| pendulum might only rotate 80% or so depending on how far north
| it was. (Easy to derive mathematically, but hard to truly intuit
| what was going on.)
|
| As I doubt I'm the only one, allow me to provide the thought
| expirement that make it click for me; specifically, why the
| pendulum turns some fraction of a complete rotation while the
| Earth -- at any point -- makes a full circuit every 24 hours.
|
| Put a pendulum in your car. If you drive straight, the pendulum
| won't be affected. If you turn, of course, the pendulum will turn
| a bit. Easy to visualize so far. Now, let's stop the Earths
| rotation for a moment. Let's get in the car and with the Pendulum
| and drive due East from L.A. All the way to N. Carolina (or
| wherever due east would end up.) Across the Atlantic. Across
| Southern Europe, Asia, Pacific, and back to L.A.
|
| Now I've been driving straight this entire time, so would the
| pendulum have moved?
|
| Well, as it runs out, I HAVEN'T been driving 'straight'. The
| entire time, I would necessarily had to have been veering a
| little bit to the left to keep me in my due East path. If I was
| truly driving straight the entire time, I would have made a Great
| Circle and dipped down into Africa at some point.
|
| In any case, as my desktop pendulum moves around the globe every
| 24 hours, it isn't traveling in a straight line... just as a car
| transporting it along its path would have to be curving a bit to
| stay on course.
|
| This was the 'ah-ha' moment that allowed to understand the
| gradual increase in the pendulums rotation as you move north (or
| south) from the Great Circle of the equator.
|
| Maybe that is common-sense for everyone else. If so, disregard.
| ;)
| aidenn0 wrote:
| TIL the smithsonian removed it's Foucault pendulum in 1998 (right
| about when I moved away from the DC area). It was about 15 meters
| long and quite impressive; they had little pins setup that it
| would knock down, so you could look at when you arrived in the
| museum and see the difference when you left.
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